CN1040949A - Blow molding method for molding flat containers having portions of widely varying wall thickness - Google Patents

Abstract

The method of producing this jerrycan is the extrusion molten resin, make its formation type, the bottom of sealing parison, the pre-blowing parison, so that the inner surface of parison is not in contact with one another, parison stretches under molten condition, make parison swell, the outward flange of parison extends the edge of mold cavity simultaneously, compress the parison of expansion with mould, along the die cavity outward flange parison outer edge part is melted in together, forces pressure fluid to enter the parison that is limited by mould so that parison with the tight contact condition compacted under of die cavity inner surface.The method can be made jerrycan easily by easy steps, even the very high container of flatness.

Description

The invention relates to a production method of a flat container. The mouth and shoulders of the container are relatively thick, while the body of the container is relatively thin and has a large degree of flatness.

Producing containers, especially flat containers with a large wall thickness in the mouth and shoulders and a small wall thickness in the container body, is quite difficult. A satisfactory and complete method has not yet been recommended. Usually such containers are produced step by step by the following various methods:

1. A method using a flat parison combined with a parison controller.

2. A method using a transverse expander (Japanese Patent Laid-Open No. 61-134224).

3. Sheet blow molding technology.

However, these methods have their own problems. Use the first method. It is difficult to produce a container with a flat mouth portion to provide a uniform wall thickness distribution on the container body. With the second method, it is difficult to obtain a uniform wall thickness distribution on the container body. With the third method, there is difficulty in achieving a uniform wall thickness distribution at the mouth of the container.

Therefore, the main object of the present invention is to eliminate these drawbacks of the prior art and to propose a flat container with a large wall thickness in the container mouth and shoulders and a small wall thickness in the container body with a section of widely varying wall thickness processing method.

This object of the present invention can be achieved by the production process of blow molding a molten resin into a flat container having a portion of greatly varying wall thickness, the process comprising:

a. Extrude the molten resin from the die head to form a parison.

b. Close the bottom of the parison with clamps.

c. Form the thick-walled part while allowing fluid with sufficient pressure to enter the parison to prevent contact between the inner surfaces of the parison.

d. Stretching the parison longitudinally at a ratio of at least 1.1 at a temperature of the parison not lower than the melting point of the resin.

e. Inflating the parison by applying differential pressure along the cross-section of the parison side wall to form the thin walled portion of the container.

f. Constraining the expanded parison in a split mold provided with a cavity adapted to the contour of said flat container, the outer edge of the parison extending beyond said cavity so that the edge portions are fused together , and make the parison into a flat shape.

g. Forcing pressurized fluid into the parison confined within said mould, so that the parison is formed in intimate contact with the inner surface of the cavity.

In a preferred embodiment, the molded container has a ratio of wall thickness of the thick wall portion to the wall thickness of the thin wall portion of at least 5.

In another preferred embodiment, the molded container has a flatness (ie, the ratio of the larger diameter to the smaller diameter of the container) of at least 2.5.

In yet another embodiment, the resin is polypropylene, polyethylene or poly-4-methylpentene-1.

Figures 1a, 1b, 1c and 1d show schematic diagrams of the four production steps of the present invention. Figure 1e is a schematic diagram showing the container produced by these steps.

Figures 2a and 2b show respectively a cross-section of a container neck and container body produced by the prior art.

Figures 3a and 3b show, respectively, a cross-section of a container neck and container body produced by the method of the invention.

Figure 4 shows the outline and dimensions of containers produced with an embodiment of the present invention.

The details of the present invention are described in more detail below. According to the present invention, molded containers can be produced using commonly used extruders and dies. Figures 1a to 1b illustrate the steps in the production of the container using a mold equipped with air nozzles 3 for inflating the extruded parison 2.

As shown in FIG. 1 a , the parison 2 is a thin tube of molten resin emerging from the mold 1 , the bottom of which is closed by means of a parison clamp 4 . Simultaneously, a certain amount of compressed air enters the parison from air nozzles 3 located in the mold 1 in order to inflate the parison 2 slightly. In other words, it prevents the inner surfaces of the parison from contacting each other to a certain extent. At this stage, the parison has the thickness needed to make the thick walled part of the container. For example the mouth and shoulders of a complete container. In Figures 1a to 1e, the parisons are shown with dashed lines for the sake of clarity.

The top of the slightly expanded parison 2 as shown in Figure 1a is constrained in a first split mold 5 as shown in Figure 1b, so that the parison is divided into two parts. A portion is designated 2c and is used to form the desired container. The other part is indicated by 2d and is removed in a subsequent process. As shown in Fig. 1b, the first mold 5 is composed of a pair of mold halves 5a. Also, the portion in contact with the parison 2 has a predetermined profile given the parison. In the situation shown in Figure 1b, the top 5b of each mold half provides a parison profile corresponding to the shape of the container mouth, as the middle to lower part 5c will provide a shape corresponding to the container shoulder. In this way, the gap between the two upper parts 5 b will become the mouth opening 6 .

After molding the mouth and shoulder of the container by means of the first mold 5, as shown in Figure 1c, the remainder of the parison 2 is still molten (because its temperature is not lower than the melting point of the resin), by Moving the parison clamp 4 stretches or pulls the parison 2 for a distance corresponding to the length of the container body. The stretching speed should be much faster than the rate at which the parison is pulled down. Simultaneously with the stretching or pulling operation, compressed air is blown into the stretched portion 2c from the nozzle 2 through the opening 6 to expand the portion 2c to a predetermined degree (pre-blowing). This stretch or draw ratio should be at least 1.1 in order to produce a greater wall thickness in the container mouth and shoulders and a smaller wall thickness in the container body.

In the next step, the second split mold 7 is brought into contact with the first split mold 5 and clamped, as shown in FIG. 1d. The second mold 7 is composed of a pair of mold halves 7a, and has a cavity 7b, the inner surface of which is in contact with the parison 2c to provide a shape corresponding to the outline of the container body.

The expanded parison will be confined within the second mold 7 in such a way that the outer edge of the parison will extend beyond the cavity 7b within the mold 7 . It is not the best way to completely confine the expanded parison inside the cavity 7b. Because if the container has a complex profile, however small, not all parts of the outer edge of the parison are simultaneously in contact with the inner surface of the cavity in successive steps of additional expansion. If the flatness of the container increases (ie: the ratio of the major diameter to the minor diameter of the cross-section of the container body, or A/B as shown in Figure 4), another disadvantage will arise. When the parison is further expanded in the cavity, the small diameter part of the parison will first contact the inner surface of the cavity, and the larger diameter part of the parison will gradually contact the inner surface of the cavity. Since the parison cannot be expanded uniformly in this way, an uneven wall thickness distribution will be produced in the parison, resulting in reduced reliability and yield of the produced container. This becomes a particularly noticeable problem when the flatness (A/B in Figure 4) of the container is 2.5 or greater. Thus, the present invention provides particular benefits for the production of containers having a flatness A/B of 2.5 or greater.

Simultaneously or after the clamping of the mold 7, compressed air is blown from the air nozzle 3 into the 2c part of the parison through the opening 6; so that the molded parison reaches the final product shape (final blowing). Then, the parison of such shape is cooled.

After complete cooling, the first mold 5 and the second mold 7 are opened, producing a container 8 with the outline shown in Figure 1e. The container 8 has a mouth 9 and a shoulder 10 formed from parts 5 b and 5 c of the first mold 5 , respectively, and a container body 11 formed from the second mold 7 . Mouth 9 and shoulder 10 are thick-walled so that they are not stretched in first mold 5 . On the other hand, the container body is thin since it is stretched and shaped against the side walls of the second mold 7 . Thus, the resulting container has a thick-walled mouth and shoulder and a thin-walled container body.

Parts 2d and 2a of the parison do not belong to the container 8 . And, the outer edge portion of the parison clamped by the second mold 7 is removed, thereby completing the production process of the container. The thickness ratio of the thick-walled part and the thin-walled part of the final product can be adjusted to 5 or more. It is quite difficult to obtain containers with such proportions with the prior art. But it is very easy to use the present invention.

Any extrudable resin can be used in the container production process of the present invention. However, polypropylene, polyethylene and poly-4-methylpentene-1 (product of the TPX trademark of Mitsui Petrochemical Industry Co., Ltd.) are preferably used. Any other general purpose resin can also be used. For molded containers with high transparency (measured by the method described in ASTM D-1238 for the melt flow value), it is best to use a resin with a high melt flow value. Using the present invention, even resin materials with a high melt flow value can be used in molded containers. Polypropylene preferably has a melt flow value not exceeding 10 g/10 min.

Any common type of extruder can be used. To prevent excessive downward stretching of the parison, it is best to use a large diameter die. If a large degree of parison expansion is required, it is best to use a small mold. When using the method of the present invention to produce flat containers, attention should be paid to all aspects of the operation including extrusion, opening and closing of molds, and parison forming.

The difference between the present invention and the traditional method in container production is mainly described below. If one were to manufacture a container with a thick-walled neck 20 as shown in Figure 2a by the conventional method, a uniform wall thickness distribution could not be obtained on the container body 21, but a thick-walled part 21a and a thin-walled part 21a as shown in Figure 2b Section 21b. To avoid such phenomena, pre-blowing is usually carried out, but only thin-walled necks can be obtained.

According to the method of the invention, the neck 20 of the parison is restrained in the first slidable mold 5, while the container body 21 is stretched and placed in a pre-blow molding state. In this way, as shown in FIG. 3b, the neck portion 20 maintains a thick-walled state, while the container body 21 obtains a thin-walled portion with uniform wall thickness distribution.

The following examples are used to further describe the present invention, but should not be considered as limiting the present invention.

As shown in Figure 4, two containers with certain contours and dimensions are produced. One is molded by the method of the present invention and the other is molded by conventional direct molding. In the direct molding method, the parison is further extruded from the second mold 7, and the parison is clamped by the first mold 5 and the second mold 7 to obtain a final given shape. The cavity of the mold has a contour corresponding to the shape of the container, and its flatness is 4.7 (A/B=4.7 as shown in Figure 4). The resin used as a raw material is polypropylene F652 with a melt flow value of 5 g/10 min.

Extrusion was carried out under the following conditions: die/core diameter ratio=70/64mm, die head temperature=220°C, screw speed 60rpm, L/D22, thread diameter=50mm, drawing or pulling at 240mm stroke and 14mm /sec rate. The pre-blowing of the expanded parison lasts for 2 seconds at a rate of 400 L/h, and the final blowing lasts for 15 seconds at a pressure of 6 kgf/cm ² . The blow molded parisons were cooled for 20 seconds to produce molded parts each weighing 40 g.

The distribution of the wall thicknesses of the two containers at various positions of the neck 20 and the container body 21 were examined. The results of the present invention are reported in Table 1, the conventional direct molding method is reported in Table 2, and the ratio of the wall thickness of each container neck to the container body wall thickness is recorded in Table 3. The wall thickness distribution of each container along the peripheral direction is statistically analyzed, and the results are recorded in Table 4.

Table 3 demonstrates that the method of the present invention can produce containers having a relatively high wall thickness ratio between the neck (spout and shoulder) and container body. Table 4 shows that the containers produced by the method of the present invention have less variation in the wall thickness of the container body.

From the foregoing, the present invention provides a simple method by which containers can be produced with a neck to body wall thickness ratio much greater than that obtained in the prior art. Furthermore, the method of the present invention can freely control the ratio of the wall thicknesses so as to provide a thin-walled container body with a uniform thickness distribution.

Claims (4)

1, a kind of production has the method for the jerrycan of big variation wall thickness part, and this method is that it may further comprise the steps with the moulding of molten resin blown-moulding:

A. from the die head extrusion melting resin, make it form parison;

B. seal the bottom of described parison with binding clasp;

C. make the thick wall part moulding, force fluid to enter parison simultaneously, stop between the parison inner surface to be in contact with one another with enough pressure;

D. be higher than under the melting point resin situation ratio longitudinal stretching parison with at least 1.1 in the parison temperature;

E. the different pressures that is used in the generation of the parison cross section of falling the wall makes described parison swell, forms the thin-walled portion of container;

F. the parison that expands is limited in the mould of splitting, this horizontal tool is provided with the die cavity that is suitable for described jerrycan profile, and the outward flange of parison extends described die cavity, so that the marginal portion is fused together, and makes parison become flat shape;

G. force pressure fluid to enter to be limited in the parison in the described mould, so as parison with the tight state of contact compacted under of die cavity inner surface.

2, in accordance with the method for claim 1, it is characterized in that: molded container thick wall part is at least 5 with the ratio of the thickness of thin-walled portion.

3, according to claim 1 or 2 described methods, it is characterized in that: the flatness of the molded container ratio of minor diameter (major diameter with) is 2.5 at least.

4, in accordance with the method for claim 1, it is characterized in that described resin is polypropylene, polyethylene or poly--4-methylpentene-1.

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